Video signal motion detecting method and noise reducer utilizing the motion

ABSTRACT

A first motion detection is performed for determining a motion or a still condition by comparing a difference signal Δ between an input video signal of a current frame and a video signal of a previous frame with a preset threshold value. A second motion detection is performed for determining a motion or a still condition by using a difference signal Δ between an input video signal of a current frame and a video signal of a previous frame at surrounding pixels and a center pixel as a target for the second motion detection. A wrong decision correction is performed for correcting an error in a result of motion detection in the first and/or second motion detections by changing a determination of a motion condition to a still condition, or a determination of a still condition to a motion condition, wherein an error is either a wrong determination of a motion condition for a still condition or a wrong determination of a still condition for a motion condition. A transition period detection is performed for detecting a motion-to-still transition period for a pixel whose data is determined as still during the correcting of a wrong decision.

BACKGROUND OF THE INVENTION

This invention relates to a method for detecting motion from a videosignal having noise or flicker superposed and a noise reducer fordecreasing a noise component utilizing a frame correlation of the videosignal and improving a signal-to-noise ratio of the video signal.

Generally, the video signal is a signal which has video informationrepeating at the periods of the frames, and there is highauto-correlation between the frames On the other hand, since the noisecomponent included in the video signal normally has littleauto-correlation, if the video signal is averaged temporally for eachframe period, the energy of the signal component hardly changes, andtherefore, only the energy of the noise component decreases, so that thenoise can be reduced. In order to obtain the above-mentioned average, aplurality of frame memories are required. Because the frame memories areexpensive, the generally practiced method is not to use a non-recursivefilter which requires a plurality of frame data but to use a one-framefirst-order recursive filter.

With regard to the noise reducer of a frame-cyclic arrangement whichreduces noise by utilizing the frame correlation of the video signal,many methods have been proposed. One of those methods which describesthe basic concept is carried in the Journal of the Institute ofTelevision Engineers of Japan Vol. 33, No. 4 (1979).

To help understand the present invention, description will first be madeof a conventional noise reducer referring to FIG. 1. In FIG. 1, an inputvideo signal 1, which comprises a component signal such as a luminancesignal or one of color difference signals, primary color signal R, G andB, is supplied to the input terminal The input video signal 1 suppliedthrough the input terminal is attenuated to (1-K) times the originalenergy by a variable attenuator 2 and becomes an input attenuated videosignal 3, which is applied to an adder 4. On the other hand, a previousvideo signal 7, which has had noise reduced and then delayed by oneframe period, is attenuated to K times the energy level held theretoforeby a variable attenuator 8 and becomes a previous-frame-attenuated videosignal 9. This video signal 9 is added with the input attenuated videosignal 3 by the adder 4, and output as an output video signal 5 from anoutput terminal, and then stored in a frame memory 6.

When the input video signal 1 is a completely still image, the frequencyspectrum of this video signal is a line spectrum with a 30-Hz period,there is no energy loss of video signal by the circuit as shown in FIG.1, and the degree of improvement in the signal-to-noise ratio can beexpressed as follows:

    Improvement of signal-to-noise ratio=10 log (1+K)/(1-K) (dB)(Eq. 1)

FIG. 2 shows changes in the improvement in the signal-to-noise ratiowith respect to coefficient K. It is obvious that the larger the K, thegreater the degree of improvement in the signal-to-noise ratio becomes.

On the other hand, generally, there is motion in the video signal, andif an image including motion is passed through the circuit in FIG. 1, anafter image persists. The time constant T of the after image is

    T=-1/(1n K) x 1/30 (sec)                                   (Eq. 2)

FIG. 3 shows after-image time constant characteristics with respect tothe coefficient K. The after-image time constant T, namely, the afterimage is larger with a larger K.

That is to say, the improvement of the signal-to-noise ratio and theoccurrence of the after image are shadows to each other. For thisreason, generally, the coefficient K is varied in the range of 0<K<1according to the motion of the input video signal. To be more specific,when the motion of the video signal is large, the K is reduced tosuppress the after image, and when the motion is small, the K isincreased, thereby increasing the degree of improvement of thesignal-to-noise ratio. This control of the K is done by a coefficientcontrol circuit 10.

From the input video signal 1 from the input terminal, the subtractersubtracts the previous frame video signal 7 which has had noise reducedand has been delayed by one frame period, and a resulting inter-framedifference signal Δ is input into the coefficient control circuit 10.The probability of the inter-frame difference signal Δ being a noisecomponent is generally high for smaller inter-frame difference signal Δ,while the probability of the inter-frame difference signal Δ being amotion component of the signal is high for larger inter-frame differencesignal Δ. Therefore, when the inter-frame difference signal Δ is small,the K is increased to increase the degree of improvement of thesignal-to-noise ratio. When the inter-frame difference signal Δ islarge, the K is decreased to suppress an occurrence of the after imageinsofar as possible.

The value of K is controlled as shown in FIG. 4 according to theinter-frame difference signal Δ input to the coefficient control circuit10.

In FIG. 4, the K is a function of the inter-frame difference signal Δ,and can be expressed by Eq. 3 below. ##EQU1##

Thus, by the conventional method, it is possible to reduce noise whileminimizing the occurrence of the after image.

On the other hand, a method for detecting motion from a noise-superposedvideo signal has been reported in ITEJ Technical Report TEBS112-1 (1986,7, 27). This method will be described with reference to FIGS. 5 and 6,and Table 1.

FIG. 5 indicates the relation between noise and a motion signal Thefrequency of zero cross of the inter-frame difference signal Δ includingnoise is considered to differ in the still-image region and in themoving-image region. By utilizing this phenomenon, the motion isdetected as follows

With the inter-frame difference signal Δ at a target pixel for motiondetection and the preset surrounding pixels around the center targetpixel in the detection range, the number of plus pixels CP and thenumber of minus pixels CN are calculated, and ξ is calculated by thefollowing equation.

    ξ=min (CP, CN)/max (CP, CN)                             (Eq. 4)

where

min (A, B) : a value of A or B whichever is smaller

max (A, B) : a value of A or B whichever is larger

In comparing ξ with a preset threshold value th (0<ξth<1), when 0≦ξ≦ξth,a decision as "motion" is made, and when ξth<ξ<1, a decision as "still"is made.

Table 1 shows a summary of correspondence between the values of ξ andthe decision results in the motion detection when the range of thesurrounding pixels, by which the decision was made, was set as 5×5pixels around the center target pixel as shown in FIG. 6 and thethreshold value was ξth=0.35. In the case of FIG. 6, since CP=16 andCN=9, ξ=0.56, so that decision was "still".

                  TABLE 1                                                         ______________________________________                                        plus     minus                                                                (CP)     (CN)             ξ decision                                       ______________________________________                                         0       25                 0 motion                                           1       24               0.04 motion                                          2       23               0.09 motion                                          3       22               0.14 motion                                          4       21               0.19 motion                                          5       20               0.25 motion                                          6       19               0.32 motion                                          7       18               0.39 still                                           8       17               0.47 still                                           9       16               0.56 still                                          10       15               0.59 still                                          11       14               0.67 still                                          12       13               0.92 still                                          13       12               0.92 still                                          14       11               0.67 still                                          15       10               0.59 still                                          16        9               0.56 still                                          17        8               0.47 still                                          18        7               0.39 still                                          19        6               0.32 still                                          20        5               0.25 motion                                         21        4               0.19 motion                                         22        3               0.14 motion                                         23        2               0.09 motion                                         24        1               0.24 motion                                         25        0                 0 motion                                          ______________________________________                                    

As described above, by the conventional motion detection method, adecision can be made as to the motion based on the inter-framedifference signal at the center pixel as the target of motion detectionand the surrounding pixels in the preset detection range.

However, in the above-mentioned conventional noise reducer, noisereduction is performed by using only a statistical feature that theinter-frame difference signal Δ is highly likely to be a noise componentwhen the inter-frame difference signal Δ is smaller, while theinter-frame difference signal Δ is highly likely to be a motioncomponent when the inter-frame difference signal Δ is larger. So, thisis not motion detection in the strict sensor of the word. By thismethod, a small motion in the inter-frame difference signal Δ is removedin the same way as noise. This removal of small motion causes an afterimage to occur in the moving image, which has been a problem.

The conventional motion detection method has another problem in which anomission of detecting the motion such as misjudging the motion as astill or an error such as misjudging the still as a motion occur veryoften owing to the effects of noise and flicker.

SUMMARY OF THE INVENTION

The first object of the present invention is provide a motion detectionmethod for detecting only a true motion with high accuracy and beinghardly effected by noise and flicker.

A second object of the present invention is to provide a noise reducerwhich suppresses the occurrence of an after image.

To achieve the above objects, a motion detection method according to thepresent invention comprises the steps of:

a first motion detection for determining motion by comparing adifference signal between an input video signal and a one-frame-delayedvideo signal with a preset threshold value and/or a second motiondetection for detecting motion by using the above-mentioned differencesignal at N×N surrounding pixels and the center pixel as the target ofmotion detection;

a wrong decision correction for correcting an error in a result ofmotion detection by the first motion detection and/or second motiondetection; and

a transition period detection for detecting a transition period of themotion from a motion detection result of the previous frame.

By this motion detection method according to the present invention, itis possible to detect with high accuracy a true motion signal from aninput video signal on which noise and flickers are superposed.

In addition, the noise reducer according to the present inventioncomprises a motion detection circuit for detecting motion from an inputvideo signal and a previous-frame video signal, which has been delayedby one frame period. By this motion detection circuit, a decision ismade as to which of a moving image part, a still image part and amotion-to-still transition part of the picture the respective pixelsrepresent, and separate noise reduction processes are performed by amoving image part, a still image part and a motion-to-still image part.The after image can be reduced to a small degree even with a motionsignal having a relatively small amplitude.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing an arrangement of aconventional noise reducer;

FIG. 2 is a characteristic diagram of the improvement in thesignal-to-noise ratio in the conventional noise reducer;

FIG. 3 is a characteristic diagram of the after-image time constant inthe conventional noise reducer;

FIG. 4 is a coefficient characteristic diagram of the coefficientcontrol circuit in the conventional noise reducer;

FIG. 5 is a signal waveform diagram for explaining the conventionalmotion detection method;

FIG. 6 is a pixel arrangement diagram for explaining the conventionalmotion detection method;

FIG. 7 is a schematic diagram showing an embodiment of the noise reduceraccording to the present invention;

FIG. 8 is a characteristic diagram of the nonlinear circuit in theembodiment of the noise reducer according to the present invention;

FIG. 9 is an explanatory diagram for explaining an embodiment of themotion detection method according to the present invention;

FIG. 10 is a pixel arrangement diagram for explaining the wrong decisioncorrecting method in the embodiment of the motion detection methodaccording to the present invention;

FIG. 11 is a frame conceptual diagram for explaining the motion-to-stilltransition period in the embodiment of the motion detection methodaccording to the present invention; and

FIG. 12 is a flowchart in the embodiment of the motion detection methodaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described with referenceto the accompanying drawings. FIG. 7 is a block diagram showing anembodiment of the noise reducer according to the present invention. InFIG. 7, reference numeral 101 denotes an input video signal comprisingcomponent signals. Reference numeral 102 denotes a subtracter (asubtraction circuit) which subtracts from the input video signal 101 anoutput signal 111 of a non-linear circuit 109 serving as a signalprocessing circuit in order to obtain an output video signal 103.Reference numeral 104 denotes a frame memory (delay circuit) fordelaying the noise-reduced output video signal 103 by one frame period.Reference numeral 106 denotes a motion detection circuit for deciding amode of the target pixel by using an input video signal 101 and aprevious-frame video signal 105, and the motion detection circuit 106controls the characteristics of the non-linear circuit 109 by adaptivecontrol according to a mode decided for each pixel. The modes producedby the motion detection circuit 106 are a moving image mode for themoving image part, a still image mode for the still image part and amotion-to-still transition period mode for the motion-to-stilltransition part. Reference numeral 107 is a subtracter (differencesignal detection circuit) which subtracts a previous-frame video signal105 from an input video signal 101 in order to obtain an inter-framedifference signal Δ 108. Reference numeral 109 designates a non-linearcircuit (signal processing circuit) for performing a non-linear processon an inter-frame difference signal Δ 108 produced by the subtracter107. Reference numeral 110 denotes a control signal output from themotion detection circuit 106 to perform the above-mentioned adaptivecontrol, while numeral 111 denotes an output signal from the non-linearcircuit 109.

Description will now be made of the operation of the above-mentionedembodiment. In this noise reducer according to the present invention, inorder to minimize an after image, which image has been a problem withconventional noise reducers, the motion detection circuit 106 is used todetect the motion in the moving image signal from an input video signal101 and a noise-reduced previous-frame video signal 105, and thecharacteristics of the non-linear circuit 109 are controlled in adaptivecontrol according to results of the motion detection to suppress theafter image in the moving image part.

More specifically, the motion detection circuit 106 detects the motionin the input video signal 101 in pixel units from a difference betweenthe input video signal 101 and the previous-frame video signal 105, anddecides modes for the individual pixels. The available modes are amoving image mode, a still image mode and a motion-to-still transitionperiod mode. To implement different kinds of signal processing accordingto modes decided, the motion detection circuit 106 controls thenon-linear circuit 109 by adaptive control according to a control signal110.

For the input/output characteristics of the non-linear circuit 109, anequation Eq. 5 for example, can be used, which is shown below. ##EQU2##

The Δ is an input to the non-linear circuit 109, and is an inter-framedifference signal derived from the input video signal 101 and thenoise-reduced previous-frame video signal 105. The φ(Δ) is an output ofthe non-linear circuit 109 evolving from the Δ. The TH is a thresholdvalue which is preset. The K₀ (0<K₀ ≦1) is a parameter which iscontrolled in adaptive control according to a result of motiondetection. In practice, various values are set for the K₀ according to amode decided by the motion detection circuit 106, that is to say, K₀ =γin the still image mode, K₀ =α in the moving image mode, and K₀ =β inthe motion-to-still transition period mode, where 0<α<β<γ<1. With largerTH and K₀, the degree of improvement in the signal-to-noise ratio isbetter, but the after image occurs more greatly.

In the noise reducer in this embodiment, a motion of the input videosignal 101 is detected, and in the still image mode, the signal-to-noiseratio is improved sufficiently by controlling the K₀ of the non-linearcircuit 109 so as to be a large value, while in the moving image mode,the K₀ is controlled so as to be a small value to minimize the afterimage. However, if there is a great difference in the degree ofimprovement of the signal-to-noise ratio between in the still image modeand in the moving image mode, a false contour appears at the borderarea, and if the false contour moves, this causes a flicker, which isvery disturbing to the viewer. Therefore, a transition period from amoving image to a still image (motion-to-still transition period) isdetected, and in the motion-to-still transition period, thesignal-to-noise ratio is improved to a degree intermediate between themoving image mode and the still image mode. To be more specific, the K₀of the non-linear circuit 109 is controlled to be a value intermediatebetween in the still image mode and in the moving image mode. By sodoing, the border part between the still image part and the moving imagepart is smoothed away, so that the motion in the picture becomesnatural.

As is clear from the foregoing description, by the noise reduceraccording to the above embodiment, the coefficient K₀ of the non-linearcircuit 109 is made small in the moving image part, so that the afterimage can be limited to a minor degree.

Referring to FIGS. 9, 10 and 11, description will then be made of themotion detection method in the motion detection circuit 106. The motiondetection circuit 106 operates in, e.g., four steps (first motiondetection, second motion detection, correcting a wrong decision, andmotion-to-still transition period detection) which are described below.The data of each pixel in the input video signal 101 is subjected to adecision by the motion detection circuit 106 to specify a mode--thestill image mode, the moving image mode or the motion-to-stilltransition period mode--by which the data is to be processed.

(1) First motion detection

A difference value Δ (i, j) is calculated as an inter-frame differenceby subtracting a pixel value PY (i, j) of the noise-reduced previousframe from a pixel value IY (i, j) of the current input signal, and theinter-frame difference is compared with TH in Eq. 5,

1) when |Δ(i, j)|>TH, the "moving image mode" is decided, and

2) when |Δ(i, j)|≦TH, the second motion detection, described below, isperformed.

This first motion detection is done by comparison with a thresholdvalue. However, when |Δ(i, j) >TH, it is no doubt possible to determinewith fairly high accuracy that the pixel concerned represents motion.For example, when the superposed noise is supposed to be Gaussian noise,even if the signal-to-noise ratio of the input image is as high as 26dB, the probability of noise level exceeding TH =40 is less then 0.3 percent.

(2) Second motion detection

In the first motion detection, when |Δ(i, j)|<TH, motion detection isperformed by using the above-mentioned difference signal Δ(i+m, j+n) ofthe N ×N surrounding pixels and the center pixel as the target of motiondetermination, where m and n are ##EQU3## when N is an even number, or##EQU4## when N is an odd number.

Description will be made of details of the step of the second motiondetection with reference to FIG. 9.

For an inter-frame difference signal Δ(i+m, j+n) (-2≦m, n≦2) at , e.g.,5×5 surrounding pixels and the center pixel as the target of motiondetermination,

the number of pixels each having a difference signal Δ(i+m, j+n) whichis greater (>) than a threshold value 2 is denoted by p₋₋ num,

the number of pixels each having a difference signal Δ(i+m, j+n) whichmeets a condition that a threshold value 1≦Δ(i+m, j+n)≦the thresholdvalue 2 is denoted by z₋₋ num,

the number of pixels each having a difference signal Δ(i+m, j+n) smaller(<) than the threshold value 1 is denoted by n₋₋ num, and decision ismade as follows.

1) when z₋₋ num≧zero₋₋ th, the decision is "still"

2) in cases other than 1), η is calculated by the following equation:

    η=min(p.sub.-- num, n.sub.-- num)/max(p.sub.-- num n.sub.-- num) (Eq. 6)

where

min (A, B) : a value of A or B whichever is smaller

max (A, B) : a value of A or B whichever is larger

the calculated η is compared with a preset threshold value ηth(0<ηth<1), and if 0≦η≦ηth, the decision is "motion"

and if ηth <η≦1, the decision is "still".

This decision method is to detect a bias of the inter-frame differencesignal Δ(i+m, j+n) to the plus or minus side, and, motion detection ismade from this bias. ηth is provided as the threshold value.

The above-mentioned threshold values 1 and 2 are provided to prevent anerror of motion detection from occurring owing to noise or flickersuperposed on the input signal.

The step of the second motion detection is followed by the step ofcorrecting a wrong decision.

(3) Correcting a wrong decision

A check is made of a result of a decision in the second motiondetection, and a decision considered faulty is corrected (correction ofa wrong decision). Correction is carried out as follows.

(3-1) Correcting a failure to detect

For the pixels which are determined as having data of a still state inthe second motion detection, the result of a decision about motion ateight surrounding pixels is checked as shown in FIG. 10, if four or moreof the eight surrounding pixels are determined as having motion data,the target pixel is forcibly treated as having motion data.

(3-2) Correcting a detection error

For the pixels determined as having motion data in the second motiondetection, the result of decision about motion at the eight surroundingpixels is checked as shown in FIG. 10, if the pixels determined ashaving motion data are two or less, the center target pixel is forciblytreated as having data of a still state.

After a wrong decision is corrected,

1) when the result of correct decision is "motion" the "moving imagemode" is decided

2) when the result of correct decision is "still"

The subsequent step of motion-to-still transition period detection isperformed.

(4) Detecting a motion-to-still transition period

For the pixel whose data is determined to be still as a result ofcorrection of a wrong decision, detection is made for a motion-to-stilltransition period in the step of detecting a motion-to-still transitionperiod. With reference to FIG. 11, description will be made of detectionof a motion-to-still transition period. With regard to a pixel of the Nframe,

1) if the result of motion detection at the corresponding pixel of theprevious N-1 frame is "moving image mode",

the "motion-to-still transition period mode" is decided

2) if the result of the above-mentioned detection is "still image mode",the "still image mode" is decided.

FIG. 12 is a flowchart showing the flow of the steps of detectionmentioned above. To be more specific, at step 121, the first motiondetection is performed, and when the pixel is found to have motion data,the "moving image mode" is decided. When the pixel is found to have dataof a still state, the second motion detection is made at step 122. Atthe next step 123, a wrong decision correction is performed, and if thecorrect decision is "motion", the "moving image mode" is decided. And,if the correct decision is "still", a motion-to-still transition periodis detected at step 124. At step 124, if the decision of previous frameis "moving image mode", the "motion-to-still transition period mode" isdecided, and if the decision of previous frame is "still image mode",the "still image mode" is decided.

In the above embodiment, description has been made of a case in whichboth the first and second motion detections are carried out, but eitherone of the two motion detections may be omitted. More specifically, itis possible to proceed to the step of wrong decision correction when|Δ(i, j)|≦TH in the first motion detection. Or otherwise, the secondmotion detection may be conducted from the beginning without doing thefirst motion detection.

The motion detection method according to this embodiment has anadvantage that this method can detect only true motion with much higheraccuracy and is hardly effected by noise or flicker than theconventional methods.

As has been described above, an effect of this motion detection methodaccording to the present invention is that even if noise or flicker issuperposed on the input video signal, only true motion can be detectedwith excellent accuracy.

In addition, an effect of the noise reducer according to the presentinvention is that this noise reducer detects true motion from the inputvideo signal with remarkable accuracy, controls a noise reductionprocess by adaptive control according to results of motion detection,can limit the after image to a minor degree even when the motion signalhas a relatively small amplitude.

What is claimed is:
 1. A motion detection method comprising the stepsof:performing a first motion detection for determining a motion or astill condition by comparing a difference signal Δ between an inputvideo signal of a current frame and a video signal of a previous framewith a preset threshold value and performing a second motion detectionfor determining a motion or a still condition by using a differencesignal Δ between an input video signal of a current frame and a videosignal of a previous frame at a center pixel as a target for said secondmotion detection and at pixels which surround said center pixel;performing a wrong decision correction for correcting an error in aresult of motion detection in said first and/or second motion detectionby changing a determination of a motion condition to a still condition,or by changing a determination of a still condition to a motioncondition, wherein an error is either a wrong determination of a motioncondition for a still condition or a wrong determination of a stillcondition for a motion condition; and performing a transition perioddetection for detecting a motion-to-still transition period for a pixelwhose data is determined as still in said step of correcting a wrongdecision.
 2. A motion detection method comprising the stepsof:performing a first motion detection for determining a motion or astill condition by comparing a difference signal Δ between an inputvideo signal of a current frame and a video signal of a previous framewith a preset threshold value and/or performing a second motiondetection for determining a motion or a till condition by using adifference signal Δ between an input video signal of a current frame anda video signal of a previous frame at a center pixel as a target forsaid second motion detection and at pixels which surround said centerpixel; performing a wrong decision correction for correcting an error ina result of motion detection in said first and/or second motiondetection by changing a determination of a motion condition to a stillcondition, or by changing a determination of a still condition to amotion condition, wherein an error is either a wrong determination of amotion condition for a still condition or a wrong determination of astill condition for a motion condition; and performing a transitionperiod detection for detecting a motion-to-still transition period for apixel whose data is determined as still in said step of correcting awrong decision; wherein if, for the difference signal Δ at each pixel inthe step of the second motion detection, the number of pixels eachhaving a difference signal Δ greater than a threshold value 2 is denotedby P₋₋ num, the number of pixels each having a difference signal Δ whichmeets a condition that a threshold value 1≦Δ≦the threshold value 2 isdenote by z₋₋ num, the number of pixels each having a difference signalsmaller than the threshold value 1 is denoted by n₋₋ num, when z₋₋ numis larger than a first specified value, a pixel data decision is "still", and in cases other than when the pixel data decision is "still", if aratio of a smaller value of p₋₋ num and n₋₋ num to a larger value of p₋₋num an dn₋₋ num is smaller than a second specified value, a pixel datadecision is "motion", and f this ratio is greater than the secondspecified value, a pixel data decision is "still".
 3. A motion detectionmethod comprising the steps of:performing a first motion detection fordetermining a motion or a still condition by comparing a differencesignal Δ between an input video signal of a current frame and a videosignal of a previous frame with a preset threshold value and/orperforming a second motion detection for determining a motion or a stillcondition by using a difference signal Δ between an input video signalof a current frame and a video signal of a previous frame at a centerpixel as a target for said second motion detection and at pixels whichsurround said center pixel; performing a wrong decision correction forcorrecting an error in a result of motion detection in said first and/orsecond motion detection by changing a determination of a motioncondition to a still condition, or by changing a determination of astill condition to a motion condition, wherein an error is either awrong determination of a motion condition for a still condition or awrong determination of a still condition for a motion condition; andperforming a transition period of detection for detecting amotion-to-still transition period for a pixel whose data is determinedas still in said step of correcting a wrong decision; wherein said stepof correcting a wrong decision comprises the step of forcibly treating acenter pixel as having data of a motion condition when more then half ofsurround pixels are determined as "motion" with said center pixel havingbeen determined a "still" in said first and/or said second motiondetection.
 4. A motion detection method comprising the stepsof:obtaining difference signals Δ between a current-frame input videosignal and a previous-frame video signal for a pixel as a target ofmotion determination and for surrounding pixels around said targetpixel; determining that the target pixel has motion data when the numberof said surrounding pixels, which meet a condition that the respectivedifference signals Δ are equal to or larger than a threshold value 1 andequal to or smaller than a threshold value 2, is less than a first valueand a ratio of whichever is smaller to whichever is larger of the numberof said surrounding pixels each having a difference signal Δ larger thanthe threshold value 2 or the number of said surrounding pixels eachhaving a difference signal Δ smaller than the threshold value 1 is lessthan a second value.
 5. A motion detection method comprising the stepsof:obtaining a difference signal Δ between an input video signal and aprevious-frame video signal for an object pixel to be subjected tomotion determination and surrounding pixels around said object pixel;obtaining a first number of pixels each having a difference signal Δsatisfying a condition of a threshold value 1≦Δ≦ a threshold value 2, asecond number of pixels each having the difference signal Δ satisfying acondition of Δ> the threshold value 2, and a third number of pixels eachhaving the difference signal Δ satisfying a condition of Δ< thethreshold value 1; and deciding that said object pixel is in a motionstate when the first number of pixels is smaller than a firstpredetermined value and a ratio of the third number of pixels to thesecond number of pixels is equal to or smaller than a secondpredetermined value, or deciding that said object pixel is in a stillstate, if the first number of pixels is greater than or equal to thefirst predetermined value.
 6. A motion detection method comprising:afirst step of comparing a difference value between an input video signaland a previous-frame video signal with a preset first value and decidinga moving image mode when said difference value is larger than said firstvalue; a second step of deciding whether data is "motion" or "still" fora pixel, for which a moving image mode has not been decided, fromdifference signals Δ of a plurality of pixels including said pixel andsurrounding pixels around said pixel; a third step of correcting aresult of the decision at the second step according to decision resultsfor said surrounding pixels and deciding a moving image mode when adecision result after correction is "motion"; and a further step forsaid pixel for which a moving image mode has not been decided in saidthird step, deciding a motion-to-still transition mode when acorresponding pixel in said previous frame is determined to have motiondata or deciding a still image mode when said corresponding pixel insaid previous frame is determined to have data of a still state.
 7. Anoise reducer comprising:a delay circuit for delaying a video signal byone frame; a difference signal Δ detection circuit for obtaining adifference signal Δ between an input video signal and a delayed videosignal; a signal processing circuit for performing signal processing onsaid difference signal Δ; a subtraction circuit for obtaining a videosignal having had noise reduced by decreasing an output signal of saidsignal processing circuit form said input video signal; a motiondetection circuit for detecting motion forms aid input video signal andsaid delayed signal; and means for adaptive control of said signalprocessing circuit according to detection results of said motiondetection circuit; wherein said motion detection circuit decides amoving image mode, a still image mode or a motion-to-still transitionmode by which data of each pixel in said input video signal is to beprocessed, and wherein the adaptive control means controls and adaptivecontrol parameter betwen 0 and 1, that is to say, sets said parameter ata small value in the moving image mode, at a large value in the stillimage mode and at a value betwen said small value and said larger valuein said motion-to-still transition mode.
 8. A noise reducer according toclaim 7, wherein said motion detection circuit includes means for,according to said input video signal and said delayed video signal,deciding said moving image mode, said still image mode or saidmotion-to-still transition mode by which data at each pixel of the inputvideo signal is to be processed.